Answer:
Tesla Motors breached the contract they had with Sarah and they are liable under the firm offer rule.
Step-by-step explanation:
The firm offer rule states that an offer shall remain open and firm until its expiration date (in this case September 15). Tesla Motors can revoke an offer (anyone can) but in order to do so, it must notify the other party about the revocation. If Tesla didn't properly revoke the offer before Sarah accepted it, then they are liable for it.